by Ting Ji
Sex-biased dispersal is a widespread phenomenon in animals and humans. Different species show marked contrasts in the patterns of sex-specific dispersal. As for humans, we use the social norms of post-marital residence to regulate sex-specific dispersal at marriage. In most human societies, women leave their natal households and live with their husband’s kin (i.e. parilocality). However, neolocal residence is more popular in industrial societies, i.e. men and women form a new family separate from the kin of either side. Less common patterns include matrilocality, where men move to wife’s household, or ambilocality, where men and women can live with either side of the kin. In a few human societies, such as Mosuo in Southwestern China, the residence is duolocal (or natallocal), where neither men nor women disperse at marriage, husband visits his wife’s household at night, and returns home in the morning. In this society, both men and women live with their close relative for their entire lives. Patterns of sex-biased dispersal structure populations and determine whether an individual is surrounded by relatives or unrelated group members, and thus play an important role in shaping social behaviors.
What predicts the differences in sex-biased dispersal patterns in humans? Anthropologists have long noticed that there is correlation between different social structures in many regions. More specifically, patrilocality is usually associated with patrilineal descent, and matrilocality is associated with matrilineal descent. In other words, in patrilineal societies, descent is traced along male lines, men are more likely than women to remain in natal households, and women are more likely to marry out. In matrilineal societies, descent is traced through female lines, the opposite pattern thus can be found. Is there, however, a causal relation between these two types of kinship norms? Murdock has argued that certain descent systems might arise from different patterns of post-marital residence, i.e., changes of residence drive the change of descent. However, phylogenetic comparative studies show that co-evolutionary trend between these two cultural traits varies in each language family. Different mating systems and local competitions have been proposed to influence sex-biased dispersal as well.
Sex-biased dispersal in humans is also affected by ecological factors. Earlier studies have described dispersal patterns vary in populations with different modes of subsistence. In many hunting, gathering, and fishing societies (foragers), there is no strict rule about which sex should remain and which sex should disperse. Couples often change their residence from time to time. While in horticulture societies, both female-biased and male-biased dispersal are found. Dispersal patterns in pastoralism and agricultural societies, however, are overwhelmingly female-biased. The association between sex-specific dispersal and mode of subsistence has led to the hypothesis that the transition from foraging to agriculture and/or animal husbandry is likely to have promoted female migration. However, the evidence is mixed, with some research finding that the emergence of intensive agriculture, plow agriculture, or large domestic animals hindered matrilcoality and other research suggesting a male-biased dispersal system during the period of the introduction and intensification of agriculture.
As one of the largest language families, the evolutionary history of sex-biased dispersal in Sino-Tibetan has attracted much attention. However, archaeological and genetic studies provide mixed conclusions, and there is still much debate. Moreover, sex-specific models are needed to fully understand the evolution of human dispersal, as post-marital residence and sex-specific dispersal are not always interchangeable. Here we studied the evolutionary history of dispersal norms for males and females in Sino-Tibetan populations, using cultural phylogenetic comparative methods. And we also tested the coevolution of sex-specific dispersal and descent and subsistence.
We collected and analyzed ethnographic data on kinship norms and subsistence from 97 Sino-Tibetan populations. As in many other places around the world, women disperse and men remain in over 85% of the studied Sino-Tibetan populations. Women do not disperse at marriage (as in matrilocal or duolocal residence) in about 5.2% of all societies, whereas approximately 9.3% of the societies lack of strict norms on whether women should disperse or not.
We found that kinship descent likely co-evolved with female dispersal, but not with male dispersal. These findings suggest that in patrilineal societies, dispersal patterns for men are quite flexible, they can choose to move or remain in their natal households. Nevertheless, there is strict control over women’s dispersal. Our results also show that the state of “patrilineal/female disperse” is very stable, and women are unlikely to change their dispersal strategy in patrilineal societies, unless descent changes first. We also found that female dispersal was likely ancestral, implying that the rare duolocal residence in Mosuo population, in which neither sexes disperse, evolved later. This result is consistent with an earlier genetic study suggesting that the Mosuo might have adopted matriloclaity recently. Furthermore, we found agriculture likely co-evolved with only female dispersal patterns, but the result is sensitive to alternative coding strategy. And we did not find association between domestic cattle and dispersal patterns of either gender. Our results thus illustrated how subsistence or descent can play different roles in shaping male and female dispersal behavior.